1. Field of the Invention
The present invention relates to techniques for reducing power consumption and was developed by paying specific attention to the possible application in optical telecommunications systems of the on-chip integrated type. However, reference to this preferred application is in no way to be construed as limiting the scope of the invention.
2. Description of the Related Art
In modern telecommunications systems, optical fibers are extensively employed as the physical layer for transmission channels. Optical fibers are in fact adapted to transmit very high frequency carriers and, by virtue of the large useful bandwidths associated to such carriers, a plurality of communication channels can be multiplexed for simultaneous transmission on a single fiber.
Optical fiber telecommunication systems are particularly simple, as they generally comprise a source such as a laser or an LED for generating modulated optical signals, at least one fiber into which the optical signals are injected to be propagated along the fiber, and a photo detector device such as a photodiode to detect the optical signals propagated along the fiber. At present, these arrangements are mostly obtained by using discrete components; however on-chip integration of those systems appears to be a promising perspective.
Such integrated systems have to cope with some basic technological issues. One of these is related to the possible need of subjecting the signal(s) to electro-optical and opto-electrical conversion to be processed. Such a conversion step inherently limits the transmission bandwidth. Moreover, attenuation introduced by the optical fiber and various noise sources play a role in making demodulation at the receiver a critical process.
In the specific context considered, operation of an optical fiber communications system generally involves multiplexing various signals streams received in parallel on an electrical bus to form a very high rate serial signal. This signal is then used to on-off modulate a light source to generate the optical signal to be injected into and propagated along the fiber.
FIG. 1 shows the typical layout of a telecommunications system embodying the principle of operation mentioned in the foregoing.
Such a system can be represented as comprising a transmitter, indicated as a whole as 10, a receiver, indicated as a whole as 20, and an optical waveguide or fiber 16 arranged therebetween.
The transmitter 10 includes a parallel-to-serial converter 12. The converter 12 receives from an electrical bus 11 an input data stream b(t) arranged over N parallel lines at a frequency (bit rate) f0, operates conversion of the signal from parallel to serial and thus generates a serial data stream at a frequency (bit rate) Nf0.
The serial bitstream is supplied to a laser driver 14 adapted to drive on optical source such as a laser diode 15. The laser diode generates an optical signal corresponding to the serial bitstream with bit rate Nf0 and adapted to be injected into the optical fiber 16.
At the other end of the optical fiber 16, the receiver 20 includes a photodetector 21, for the opto-electrical conversion of the optical signal propagated along the fiber 16, followed by an amplifier 22 and a comparator (threshold detector) 23.
Downstream of the detector 21 a (virtually) identical replica of the serial signal supplied to the optical source 15 is produced.
The signal in question being designated “virtually” identical is obviously intended to take into account the error intrinsically involved in the transmission/detection process.
The signal from the comparator 23 is sent to a serial-to-parallel converter 24 acting essentially as a de-multiplexer adapted to derive—from the serial bitstream received with bit rate Nf0—a number N of parallel bitstreams each with bit rate f0 that virtually correspond to the input data stream b(t).
From the parallel-to-serial converter 12 a clock signal CK at a frequency Nf0 is derived to be fed to an optical source such as a laser diode 15′ driven by a corresponding laser driver 14′ to be injected into an optical fiber 16′. At the output end of the fiber 16′, the optical signal is received by a photodetector 21′, followed by an amplifier 22′ and a comparator 23′ and supplied to the clock input of the serial to parallel converter 24, in order to drive in a correct way the operation of conversion.
Those of skill in the art will appreciate that the fibers 16, 16′ may in fact be the same fiber with the optical source/photodetector pair 15, 21 operating at a different wavelength with respect to the optical source/photodetector pair 15′, 21′, according to what is currently known as a WDM (Wavelength Division Multiplex) format.
Data on the electrical bus 11 are transmitted on an integer number N of lines. Therefore, parallel-to-serial conversion leads these data to be converted to a frequency N times the frequency of the clock signal associated to the electrical bus 11. The clock signal CK, at a frequency Nf0, is transmitted on the optical fiber 16′ to permit proper recovery of the data in the reception stage by ensuring the correct synchronization of the detection process.
In the arrangement described, the modulation format adopted for the optical source 15 is of an on-off or OOK (On/Off Key) type.
FIG. 2 shows an exemplary behaviour as a function of time t of the optical power P0(t) impinging on the photodetector 21, while FIG. 3 shows the corresponding time behaviour of the current l(t) at the output of the photodetector 21. The logical levels of the input signal are “0” and “1”.
The modulated optical signal power P0(t) can be expressed as:
                                          P            0                    ⁡                      (            t            )                          =                              P            M                    ·                                    ∑              k                        ⁢                                          b                k                            ·                              p                ⁡                                  (                                      t                    -                    kT                                    )                                                                                        (        1        )            where PM indicates the power emitted by the laser source, bk is a binary coefficient and p(t) the signal expressing the pulse shape. This is therefore a baseband PAM modulation (Pulse Amplitude Modulation), where the elementary ideal impulse response is constituted by a rectangular impulse.
Therefore, in a system as shown in the foregoing the laser source is switched on in correspondence to high logical states of the signal to be modulated.
In bus technology, as currently adopted e.g. in microprocessor-based processing systems, various solutions are known that reduce switching activity on the electrical bus.
For instance, in European patent application 02425456.7, a so-called “bus invert” method is disclosed based on the following operating principle.
If b(t) indicates the input data stream, the coded data stream B(t) used for transmission over the bus is generated according to the relationship:
                              [                                    B              ⁡                              (                t                )                                      ,            INV                    ]                =                  {                                                                                          b                    ⁡                                          (                      t                      )                                                        ,                  0                                                                                                  H                    ⁡                                          [                                                                        b                          ⁡                                                      (                            t                            )                                                                          ⊕                                                  B                          ⁡                                                      (                                                          t                              -                              1                                                        )                                                                                              ]                                                        <                                      N                    /                    2                                                                                                                                                                  b                      ⁡                                              (                        t                        )                                                              _                                    ,                  1                                                            otherwise                                                                        (        2        )            where H is a Hamming distance function for counting the transitions involved in passing from B(t−1) to B(t), and INV is a signal transmitted on an additional line to inform the receiver as to whether the data transmitted are encoded or not.
The technique in question therefore measures a number of switching events—called switching activity (SA)—that would take place if the data without coding were transmitted. If such switching activity SA is lower than N/2, the input data stream b(t) is transmitted without encoding; otherwise its inverted value is transmitted. Such technique guarantees that the switching activity SA on the bus is always lower than N/2.
Another solution known in connection to electrical buses it is a method called ‘bus switching’.
This is essentially an integrated technique of swapping and coding the input data stream b(t). The parallel electrical bus having N lines is divided in a plurality of identical “clusters” of M lines each. The bus switching method provides for choosing an optimal sequence of swapping p that minimizes the total switching activity SA. The encoder output i.e. the coded data stream B(t), is function of the input data stream b(t) and of the optimal sequence of swapping p, through a swapping operator S:B(t)=S(b(t), p)⊕S−1(B(t−1), p)  (3)
The swapping operator S applies the sequence of swapping to every cluster of the parallel bus.